1. Field of the Invention
The present invention relates to a porous glass substrate for displays and a method of manufacturing the same, and more particularly, to a porous glass substrate for displays and a method of manufacturing the same, with which the optical characteristics of a display such as an organic light-emitting device (OLED) can be improved.
2. Description of Related Art
FIG. 6 is a cross-sectional and conceptual view depicting the light extraction efficiency of an organic light-emitting device (OLED) of the related art. As shown in FIG. 6, in the OLED of the related art, only about 20% of light generated is emitted to the outside and about 80% of the light is lost by waveguiding due to the different refractive indices of a glass substrate 10, an anode 20 and an organic light-emitting layer 30 which includes a hole injection layer, a hole carrier layer, a light-emitting layer, an electron carrier layer and an electron injection layer and by total internal reflection due to the difference in the refractive indices between the glass substrate 10 and air. Specifically, the refractive index of the inside organic light-emitting layer 30 ranges from 1.7 to 1.8, and the refractive index of indium tin oxide (ITO) that is typically used for the anode 20 ranges from 1.9 to 2.0. The two layers have a very small thickness ranging from about 100 nm to about 400 nm and the refractive index of glass which is used for the glass substrate 10 is about 1.5, whereby a planar waveguide is formed inside the OLED. It is calculated that about 45% of the light is lost by the inside waveguiding due to the above-described reason. In addition, the refractive index of the glass substrate 10 is about 1.5, and the refractive index of the ambient air is 1.0. When the light is directed outward from the inside of the glass substrate 10, a ray of the light having an angle of incidence greater than a critical angle is totally reflected and is trapped inside the glass substrate 10. The ratio of the trapped light is up to about 35%, and thus only about 20% of the generated light is emitted to the outside. Herein, reference numerals 31, 32 and 33 indicate components of the organic light-emitting layer 30. Specifically, 31 indicates the hole injection layer and the hole carrier layer, 32 indicates the light-emitting layer, and 33 indicates the electron injection layer and the electron carrier layer.
In the related art, in order to overcome this problem, a silica aerogel film, or a low-refractivity film, is situated as a coating between the glass substrate and the ITO in order to extract light that is trapped by total internal reflection (see FIG. 7). However, the silica aerogel has not been applied to an actual product, since the manufacturing process thereof is complicated, difficult and expensive. In addition, the ability of the silica aerogel to be formed in a thin film is limited, thereby causing the increased thickness of the substrate.
In addition, as shown in FIG. 8, in the related art, a concave-convex structure 60 is disposed under the anode 20 (with respect to the paper surface), i.e. in the interface between the anode 20 and the glass substrate 10, in order to enhance light extraction efficiency.
As described above, the anode 20 and the organic light-emitting layer 30 generally act as one light waveguide between the cathode 40 and the glass substrate 10. Accordingly, in the state in which the anode 20 and the organic light-emitting layer 30 act in a waveguide mode, when the concave-convex structure 60 which causes light scattering is formed in the interface adjacent to the anode 20, the waveguide mode is disturbed, so that the quantity of light that is extracted to the outside is increased. However, when the concave-convex structure 60 is formed below the anode 20, the shape of the anode 20 resembles the shape of the concave-convex structure 60 below the anode 20, thereby increasing the possibility that a sharp portion may be localized. Since the OLED has a stacked structure of very thin films, when the anode 20 has a sharp protruding portion, current is concentrated in that portion, which acts as a reason for large leakage current or decreases power efficiency. Accordingly, in order to prevent such deterioration in the electrical characteristics, a flat film 70 is necessarily added when the concave-convex structure 60 is formed below the anode 20. The flat film 70 serves to make the convex and concave portions of the concave-convex structure 60 be flat. When the flat film 70 is not flat and has sharp protruding portions, the anode 20 also has protruding portions, which cause leakage current. Therefore, the flatness of the flat film 70 is very important and is required to be about Rpv=30 nm or less.
In addition, the flat film 70 is required to be made of a material, the refractive index of which is similar to that of the anode 20. If the refractive index of the flat film 70 is low, most light is reflected at the interface between the anode 20 and the flat film 70 before being disturbed by the concave-convex structure 60. The light is then trapped between the anode 20 and the organic light-emitting layer 30, which is referred to as the waveguide mode. The flat film 70 is required to be as thin as possible. If the flat film 70 is too thick, more light may be unnecessarily absorbed, and the effect of scattering may be decreased since the distance between the concave-convex structure 60 and the organic light-emitting layer 30 is too large.
However, the process of completely flattening the concave-convex structure 60 using the thin flat film 70 having a thickness of several hundreds of nm is very difficult. In addition, the methods of covering and flattening the concave-convex structure 60 include deposition coating and solution coating. Since the deposition coating is characterized by forming a film following the shape of the concave-convex structure 60, the solution coating is better than the deposition coating when forming the flat film 70. However, at present, it is very difficult to obtain a solution coating material that has a high refractive index, i.e. a refractive index that is equal to or greater than the refractive index of the ITO anode 20, and that satisfies process conditions for polycrystalline thin-film transistors, such as complicated conditions required on the surface of the OLED substrate and high-temperature processing.
In the related art, a micro-cavity structure was applied to an OLED in order to improve the light extraction efficiency of the OLED. Here, the ITO anode 20, or the transparent electrode, is made of ITO/metal/ITO. In this approach, a part of light is reflected from the anode 20 and micro cavities are formed between the anode 20 and the metal cathode 40 such that the light is subjected to constructive interference and resonance, thereby increasing the light extraction efficiency. However, this micro-cavity structure has the problem of causing color shift in which colors are changed depending on the position of a viewer within the viewing angle.
The information disclosed in the Background of the Invention section is only for better understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.